As modern electronic circuit boards evolve toward increased circuit and component densities, thorough cleaning of the boards after soldering becomes more important. Current industrial processes for soldering electronic components to circuit boards involve coating the entire circuit side of the board with a flux and thereafter passing this coated side of the board over preheaters and through molten solder. The flux cleans the conductive metal parts and promotes adhesion of the solder. Commonly used fluxes consist, for the most part, of rosin used alone or with activating additives such as amine hydrochlorides or oxalic acid derivatives.
After soldering, which thermally degrades part of the rosin, the flux and flux residue are often removed from the board with an organic solvent.
Current industrial processes for cleaning the circuit boards after soldering involve the use of vapor defluxing techniques. In the conventional operation of a vapor defluxer, the board is passed through a sump of boiling organic solvent which removes the bulk of the rosin (including thermally degraded rosin) and thereafter through a sump containing freshly distilled solvent near room temperature, and finally through solvent vapor over a boiling sump which provides a final rinse with clean solvent which condenses on the circuit board. In addition, the board can also be sprayed with distilled solvent before the final rinse.
It can be seen, therefore, that the requirements of a solvent to be used in cleaning circuit boards are very stringent. Such a solvent should have a relatively low boiling point, be nonflammable, have low toxicity, and exhibit high solvency for flux and flux residue.
Ideally, such a solvent would be a single pure solvent, but in practice it has not been possible to provide such a single solvent with the above-mentioned desired characteristics. Therefore in the art, it has been the practice to use a mixture of solvents to control boiling, flammability and solvent power characteristics.
While solvent mixtures may be carefully designed to effectively control boiling, flammability and solvent power characteristics, such a solvent mixture is not necessarily useful in the industrially used circuit board cleaning procedure such as the vapor defluxing technique as described above. The major deterrent to the use of such solvent mixtures is that of fractionation to an undesirable degree during use. For example, in the vapor defluxing technique described above, the first stage of cleaning consists of passing the circuit board into a sump of boiling organic solvent under which conditions the lower boiling component of the solvent mixture may be vaporized leaving behind a solvent mixture with altered characteristics. Again in this cleaning procedure, the final rinsing is carried out by passing the cleaned circuit board through solvent vapors over a boiling sump which provides the final rinse with clean pure solvent which condenses on the circuit board wherein any fractionation may alter the solvency characteristics. And finally, for the cleaning procedures to be economically viable, the used solvent must be readily recovered for reuse which in the case of liquid solvent is usually by distillation and then there must be an assurance that the recovered solvent has the same composition and characteristics as the original solvent system.
On the other hand, azeotropic mixtures, with their constant boiling and constant composition characteristics, have been found to be very useful. Azeotropic mixtures exhibit either a maximum or minimum boiling point and do not fractionate upon boiling. These characteristics are also important in the use of the solvent compositions to remove solder flux and flux residue from printed circuit boards. Preferential evaporation of the more volatile components of the solvent mixtures, which would be the case if they were not azeotropes, or azeotrope-like, would result in mixtures with changed compositions which may have less desirable properties, such as lower solvency for rosin flux, less inertness toward the electrical components and increased flammability. These are also desirable in vapor degreasing operations where redistilled material is usually used for final rinse-cleaning. Thus, the vapor defluxing or degreasing system acts as a still. Unless the solvent composition exhibits a constant boiling point, i.e. is an azeotrope or is azeotrope-like, fractionation will occur and undesirable solvent distribution may act to upset the safety and effectiveness of the cleaning operation.
Unfortunately, as recognized in the art, it is not possible to predict the formation of azeotropes and this obviously complicates the search for new azeotropic systems which have application in this field. Nevertheless, there is a constant effort in the art to discover new azeotropes or azeotrope-like systems which have desirable solvency characteristics and particularly a greater versatility of solvency power.
One organic solvent found to be useful in the circuit board cleaning art is 1,1,2-trichloro-1,2,2-trifluoroethane (CCl.sub.2 FCClF.sub.2), which may be designated as CFC-113, because of its nonflammability, low toxicity and inertness to the components of the circuit boards. To increase the flux-dissolving ability of CFC-113, it has been suggested that more active solvents for flux, such as lower alcohols, be added.
A number of fluorocarbon based azeotropic compositions have been discovered and in some cases used as solvents for the removal of solder flux and flux residue from printed circuit boards and for miscellaneous vapor degreasing applications. For example, U.S. Pat. Nos. 3,960,746 and 3,455,835 disclose azeotropic-like mixtures of 1,1,2-trichloro-1,2,2-trifluoroethane and trans-1,2-dichloroethylene and U.S. Pat. No. 2,999,816 discloses a binary azeotrope of 1,1,2-trichloro-1,2,2-trifluoroethane and methanol.
It is an object of the present invention to provide a nonflammable azeotrope or azeotrope-like solvent composition useful for solvent cleaning applications.